Low profile platform scales for weighing relatively heavy loads are characterized by having a relatively small overall height as measured from the floor or other scale-support surface to the top of the load-receiving platform or deck. Scales of this type are especially suitable for weighing loads which are carted by such vehicles as tote bins, push carts, dollies, fork lift trucks and other usually relatively small motor vehicles.
Low profile type platform scales normally do not require installation in a pit to accommodate load-carrying vehicles and roll-on loads in general because the platform is relatively close to the floor surface and can easily be accessed by relatively short, gently sloped ramps. Thus, low profile type platform scales offer the significant advantage of pitless installation. On the other hand, in installations where it is desired to locate the platform flush with the floor surface, only a shallow pit is needed.
Various structural features contribute to the relatively high heights of the platform in prior non-low profile type vehicle platform scales to make them unsuitable for handling roll-on or wheel-on loads without installing the scale in a relatively deep pit.
In earlier versions of vehicle platform scales, platform-supporting lever systems contribute greatly to the overall height of the scale. In later versions which employ load cells to directly support the load-receiving platform, the overall scale height may be reduced somewhat, but the need arises for horizontal restraint systems and particularly for systems for minimizing side loading of the platform-supporting load cells. These systems, as heretofore constructed, are generally relatively high and would contribute significantly to the overall ground-to-platform height in a low profile type platform scale.
Load cell side loading impairs the accuracy of weight measurements and is the result of applying horizontal force components to the load cell. Factors contributing to load cell side loading mainly are temperature-induced expansion and contraction of the platform and horizontal shock loads which result from the application of rolling loads to the platform.
One example of a vehicle platform scale in which the platform is typically supported by load cells at the corners of the platform is described in U.S. Pat. No. 3,587,761 which issued on June 28, 1971 to L. C. Merriam et al. In this patent the structure for effectively preventing side loading of the load cells is vertically interposed between each load cell and the platform to significantly increase the overall ground-to-platform height of the scale.
Various proposals have been made in the past to reduce the ground-to-platform height to a low profile stature. One prior arrangement, which is frequently used, mounts the load cells outboard of the load-receiving platform in side curb housings. These side curb housings are significantly higher than level of the platform itself and customarily extend the full length of the platform on opposite sides thereof.
Such side curb housings are objectionable because they constitute obstructions which block access to the platform except from opposite ends of the scale. In comparison with the inboard arrangement where the load cells are under and directly support the platform, the outboard arrangement of the load cells additionally requires a special lever or flexure system extending between the platform and the floor to transfer the load-induced forces from the platform to the load cells. Such lever and flexure systems govern the ground-to-platform height of the scale and add significantly to the manufacturing cost thereof.
Apart from low profile considerations, known prior horizontal restraint systems in platform type scales, which are suitable for accommodating roll-on loads, are typically of the redundant or over-restraining type. A redundant and hence statically indeterminate restraint system is, in general, one which develops more than the minimum required number of reactive components to counteract all externally applied forces exerted on a free body to keep the free body stable or in equilibrium.
In platform scales, a redundant restraint system creates a problem in that strains developing from thermal expansion of the platform and other factors cause objectionable side loading of the platform-supporting load cells, and load cell side loading, as previously mentioned, induces errors in the weight measurements.
Examples of prior platform and other vehicle scales are described in U.S. Pat. No. 3,587,761 issued on June 28, 1971 to L. C. Merriam et al., U.S. Pat. No. 3,103,984 issued on Sept. 17, 1963 to C. L. Ellis et al., U.S. Pat. No. 3,266,585 issued on Aug. 16, 1966 to C. D. Boadle, U.S. Pat. No. 3,082,834 issued on Mar. 26, 1963 to C. L. Ellis, U.S. Pat. 2,793,851 issued on May 28, 1957 to A. C. Ruge, U.S. Pat. No. 2,962,276 issued on Nov. 29, 1960 to A. L. Thurston, U.S. Pat. No. 3,299,976 issued on Jan. 24, 1967 to C. D. Boadle et al., U.S. Pat. No. 3,565,196 issued on Feb. 23, 1971 to E. Laimins et al., U.S. Pat. No. 3,679,011 issued on July 25, 1972 to I. M. Hawver, U.S. Pat. No. 3,526,287 issued on Sept. 1, 1970 to R. Flinth, and German Pat. No. 673,395 (Ausgegeben Mar. 21, 1939).